There is a critical need to delineate pathobiological correlates of chronic GVHD (cGVHD) and to develop new strategies to prevent and treat cGVHD. These strategies must control both acute and cGVHD, minimize risk of opportunistic infection, and maintain GVL. To do this our understanding of GVHD pathobiology must be broadened. For instance, recognizing that antibody responses to HY antigens are important in cGVHD physiology underscores the need to develop more tools to identify somatic mHA as a step toward the integration of mHA and genetic determinates of immune reactivity. Thus, a coordinated effort combining 1) clinical trials, 2) the development of a relevant animal model to study pathobiology and in which new therapies can be tested, and 3) further elucidation of the immunologic basis of human cGVHD is essential. Project 1 will develop strategies to prevent cGVHD by targeting B cells and regulatory T cells. The first strategy is a novel anti-CD20 prophylaxis regimen, while the second strategy will allow the expansion of regulatory T cells by eliminating calineurin inhibitors from GVHD prophylaxis. The second aim is to prevent hepatic VOD by adjusting the therapy to avoid hepatic injury or by preventing vascular complications using the novel agent, defibrotide. Finally, two complementary approaches to treat steroid-resistant GVHD will be assessed: 1) coordinate inhibition of both B and T cells, and 2) expansion of regulatory T cells with ultra low dose IL-2. Project 2 will expand its studies of a new, clinically relevant mouse model of cGVHD. This will allow the pathobiology of cGVHD to be explored in the first aim, while the second aim models therapeutic approaches that can be clinically assessed in Project 1. Project 3 will continue its work in minor histocompatitbility antigen discovery moving now from sex-chromosome associated proteins to more generalized antigen discovery. This project will also develop GVHD-specific biomarkers that will allow more precise prediction of who will develop cGVHD as well as allow the therapy to be closely monitored to reduce toxicity. Ultimately we envision an integrated genomic profile that will determine the type of GVHD prophylaxis that will be most effective.